Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 361: A patient is being evaluated for an electrolyte imbalance. Which electrolyte is most critical for the transmission of nerve impulses?

A. Calcium
B. Magnesium
C. Potassium
D. Sodium (Correct Answer)

Explanation: ***Sodium*** - **Sodium ions** play a crucial role in establishing and maintaining the **resting membrane potential** in neurons, and their rapid influx through voltage-gated channels is essential for the **depolarization phase of an action potential**. - The movement of sodium across the neuronal membrane drives the **propagation of nerve impulses** along the axon. *Calcium* - While essential for **neurotransmitter release** at the synaptic terminal, **calcium** is not the primary electrolyte responsible for the creation and propagation of the action potential along the nerve fiber itself. - Its main role in nerve transmission is modulating the exocytosis of vesicles containing neurotransmitters. *Magnesium* - **Magnesium** is an important cofactor for many enzymes and plays a role in stabilizing membranes, but it does not directly participate in the rapid changes in membrane potential that characterize nerve impulse transmission. - It can act as a **calcium channel blocker** and influence neurotransmitter release, but it's not the primary ion for impulse generation. *Potassium* - **Potassium ions** are crucial for the **repolarization phase** of the action potential, moving out of the cell to restore the resting membrane potential. - Although vital for resetting the neuron, potassium's primary role is not in the initial firing or depolarizing phase of the nerve impulse.

Question 362: A 30-year-old female presents with muscle weakness and fatigue. Her serum calcium levels are elevated. Which of the following is a likely physiological consequence of hypercalcemia?

A. Decreased neuromuscular excitability (Correct Answer)
B. Decreased neuronal conduction velocity
C. Increased risk of hypocalcemia
D. Increased neuromuscular excitability and tetany

Explanation: ***Decreased neuromuscular excitability*** - Elevated **calcium levels** stabilize the neuronal cell membrane, making it **less permeable to sodium ions**. - This reduces the likelihood of depolarization and action potential generation, leading to **muscle weakness** and **fatigue**. *Increased risk of hypocalcemia* - **Hypercalcemia** refers to elevated calcium levels, so an increased risk of hypocalcemia (low calcium levels) is contradictory. - The body's regulatory mechanisms would try to decrease calcium in response to hypercalcemia, not cause hypocalcemia. *Decreased neuronal conduction velocity* - While hypercalcemia can affect neuronal function, it primarily **decreases excitability** rather than directly slowing conduction velocity along the axon. - The main impact is on the threshold for firing an action potential. *Increased neuromuscular excitability and tetany* - **Increased neuromuscular excitability** and **tetany** are characteristic symptoms of **hypocalcemia** (low calcium), not hypercalcemia. - In hypocalcemia, the neuronal membrane becomes more permeable to sodium, leading to spontaneous depolarization and muscle spasms.

Question 363: A patient presents with muscle cramps and tetany. Laboratory results show hypocalcemia. Which of the following physiological mechanisms is most likely responsible for these symptoms?

A. Increased binding of calcium to albumin
B. Decreased release of parathyroid hormone
C. Decreased neuronal excitability
D. Increased permeability to sodium (Correct Answer)

Explanation: ***Increased permeability to sodium*** - **Hypocalcemia** reduces the threshold for excitation, making nerve and muscle cells **hyperexcitable**. - This occurs because fewer calcium ions stabilize the cell membrane, leading to an easier influx of **sodium ions** and subsequent depolarization. *Decreased neuronal excitability* - This is incorrect as **hypocalcemia** actually leads to **increased neuronal excitability**, causing symptoms like tetany and cramps. - Reduced extracellular calcium increases membrane permeability to sodium, making it easier for neurons to fire action potentials. *Increased binding of calcium to albumin* - While increased binding of calcium to albumin (e.g., in alkalosis) can lead to **hypocalcemia**, it describes a cause of hypocalcemia, not the physiological mechanism directly responsible for the symptoms of muscle cramps and tetany. - The direct mechanism causing symptoms relates to the *effect* of reduced free calcium on nerve and muscle cell membranes. *Decreased release of parathyroid hormone* - **Decreased PTH release** is a common *cause* of hypocalcemia (e.g., hypoparathyroidism), but it is not the direct physiological mechanism behind the muscle cramps and tetany. - PTH regulates calcium levels but does not directly mediate the effect of low calcium on nerve and muscle cells.

Question 364: A patient exhibits hyperkalemia. Which of the following explains how this condition affects nerve excitability?

A. Increases resting membrane potential (Correct Answer)
B. Increases action potential threshold
C. Decreases action potential threshold
D. Decreases membrane potential

Explanation: ***Increases resting membrane potential (makes it less negative/depolarized)*** - **Hyperkalemia** (elevated extracellular K⁺) **depolarizes the resting membrane potential**, making it **less negative** (e.g., from -70 mV to -60 mV). - This occurs because the **Nernst equilibrium potential for K⁺** becomes less negative when extracellular K⁺ increases, shifting the resting potential closer to 0 mV. - The membrane potential moves **closer to the threshold** (which remains constant at ~-55 mV), **initially increasing excitability**. - However, prolonged depolarization causes **inactivation of voltage-gated Na⁺ channels**, leading to **paradoxical decreased excitability**. - Note: The term "increases" here means the membrane potential becomes **less negative** (moves toward 0 mV), not that it becomes more polarized. *Increases action potential threshold* - The **action potential threshold remains constant** at approximately -55 mV and does **not change** with hyperkalemia. - What changes is the **resting membrane potential**, not the threshold. *Decreases action potential threshold* - The **threshold potential does not decrease** in hyperkalemia; it remains fixed at ~-55 mV. - The misconception arises from confusing the **gap between resting potential and threshold** (which decreases) with the **threshold itself** (which stays constant). - While the membrane potential moves closer to threshold, the threshold value itself is unchanged. *Decreases membrane potential* - This phrasing is ambiguous. If "decreases" means becoming **more negative** (hyperpolarization), this is incorrect—hyperkalemia causes **depolarization** (less negative). - If "decreases" means the **absolute value decreases** (e.g., from -70 mV to -60 mV, moving toward 0), this could be correct but is poorly worded. - The preferred terminology is that hyperkalemia **depolarizes** the membrane or makes it **less negative**.

Question 365: What role does the sodium-potassium pump play in neuronal activity?

A. It generates action potentials
B. It directly generates the depolarization phase
C. It restores resting membrane potential after action potentials (Correct Answer)
D. It synthesizes neurotransmitters

Explanation: ***It restores resting membrane potential after action potentials*** - The **sodium-potassium pump** actively transports **3 Na+ ions out** of the cell and **2 K+ ions into** the cell against their concentration gradients, utilizing ATP. - This action helps to re-establish the **negative resting membrane potential** by maintaining the electrochemical gradient disrupted during an action potential. - The pump is essential for **long-term maintenance** of ionic gradients that allow neurons to fire repeatedly. *It generates action potentials* - **Action potentials** are primarily generated by the rapid influx of **Na+ ions** through **voltage-gated sodium channels**, not by the sodium-potassium pump. - The pump's role is to maintain the gradient necessary for these channels to function, not to initiate the depolarization phase itself. *It directly generates the depolarization phase* - The **depolarization phase** of an action potential is caused by the rapid opening of **voltage-gated sodium channels**, allowing Na+ to flow down its concentration gradient into the cell. - The sodium-potassium pump works **slowly and continuously** (pumps ~200 ions/second), whereas action potential depolarization occurs in **milliseconds** through passive ion flow. - The pump maintains the **gradient** that makes depolarization possible but does not directly cause it. *It synthesizes neurotransmitters* - **Neurotransmitter synthesis** primarily occurs in the neuronal cytoplasm or nerve terminals through various enzymatic pathways and is not a direct function of the sodium-potassium pump. - The pump is involved in **ion transport** and **maintaining membrane potential**, which are crucial for neuronal excitability and signaling, but not for manufacturing neurotransmitter molecules.

Question 366: A 40-year-old man with a history of alcohol abuse presents with ataxia and confusion. What is the most likely physiological basis for his symptoms?

A. Thiamine deficiency (Correct Answer)
B. Hypocalcemia
C. Hyperkalemia
D. Hyponatremia

Explanation: ***Thiamine deficiency*** - Chronic alcohol abuse often leads to **malnutrition** and impaired absorption of **thiamine (vitamin B1)**, which is crucial for neurological function. - Deficiency manifests as **Wernicke-Korsakoff syndrome**, characterized by the triad of **ataxia**, **ophthalmoplegia**, and **confusion**, as well as memory impairment. *Hypocalcemia* - While it can cause neurological symptoms, these typically include **tetany**, **seizures**, and **paresthesias**, rather than primarily ataxia and confusion. - Hypocalcemia is less directly linked to chronic alcohol abuse as the primary cause of this specific symptom complex. *Hyperkalemia* - This electrolyte imbalance primarily affects **cardiac rhythm** and muscle function, leading to **muscle weakness** and potential **cardiac arrest**. - It does not commonly present with prominent ataxia and confusion as its main neurological manifestations. *Hyponatremia* - Can cause neurological symptoms ranging from mild **confusion** to **seizures** and **coma** due to **cerebral edema**. - While confusion is a feature, ataxia is not a primary symptom, and the symptom complex in this case is more indicative of thiamine deficiency.

Question 367: A patient with hyperkalemic periodic paralysis experiences muscle weakness. How does this condition affect muscle excitability?

A. Shifts sodium channel inactivation kinetics
B. Depolarizes RMP, leading to muscle fiber inexcitability (Correct Answer)
C. Depolarizes RMP, reducing sodium channel availability
D. Hyperpolarizes RMP, impairing action potential generation

Explanation: ***Depolarizes RMP, leading to muscle fiber inexcitability*** - In **hyperkalemic periodic paralysis**, episodes of muscle weakness are triggered by elevated plasma potassium levels. High extracellular K+ **depolarizes the resting membrane potential (RMP)**, making it less negative (e.g., from -90 mV to -60 mV, closer to threshold). - While this initial depolarization might cause brief hyperexcitability, it subsequently leads to **sustained inactivation of voltage-gated sodium channels**. Sodium channels enter an inactivated state at depolarized potentials and cannot be activated until the membrane repolarizes. - This renders the muscle fibers **inexcitable** and unable to generate action potentials, causing **flaccid paralysis**. - The key mechanism is: **chronic depolarization → Na+ channel inactivation → loss of excitability**. *Hyperpolarizes RMP, impairing action potential generation* - **Hyperpolarization** means the membrane becomes more negative (e.g., from -90 mV to -100 mV), moving **further from threshold**. - While hyperpolarization does reduce excitability by requiring larger stimuli to reach threshold, this is **not what occurs in hyperkalemia**. - Hyperkalemia causes **depolarization** (less negative RMP), not hyperpolarization. *Shifts sodium channel inactivation kinetics* - While **sodium channel inactivation** is indeed central to the pathology, this option is too vague and doesn't explain the underlying cause. - The complete mechanism requires understanding that **depolarization of the RMP** drives sodium channels into an inactivated state. - This option lacks the critical detail about **how the RMP changes** (depolarization) and the consequence (inexcitability). *Depolarizes RMP, reducing sodium channel availability* - This option is **physiologically accurate** and describes part of the mechanism correctly. - **Depolarization** does indeed lead to **reduced sodium channel availability** by promoting the inactivated state. - However, it is less comprehensive than the correct answer because it doesn't explicitly state the **ultimate consequence: muscle fiber inexcitability and paralysis**. - The designated correct answer better captures the complete pathophysiological sequence and clinical outcome.

Question 368: Which type of muscle contraction involves the shortening of the muscle while generating force?

A. Isometric
B. Concentric (Correct Answer)
C. Eccentric
D. Isokinetic

Explanation: ***Concentric*** - **Concentric contraction** occurs when the muscle shortens under tension, overcoming the resistance. - This type of contraction is responsible for moving a load or accelerating a body segment. *Isometric* - **Isometric contraction** involves muscle activation without a change in muscle length; the force generated by the muscle equals the load. - An example is holding a heavy object steady in one position, where no movement occurs. *Eccentric* - **Eccentric contraction** occurs when the muscle lengthens while under tension, often acting as a brake against an external force. - This type of contraction typically happens during the controlled lowering of a weight or decelerating a body segment. *Isokinetic* - **Isokinetic contraction** involves muscle contraction at a constant velocity throughout the range of motion, requiring specialized equipment to maintain this speed. - The force generated by the muscle varies to keep the speed constant, typically seen in rehabilitation settings.

Question 369: Partial ptosis due to oculomotor nerve injury is due to intact what?

A. Supply from opposite oculomotor nerve
B. Sympathetic innervation (Correct Answer)
C. Parasympathetic innervation
D. Action of orbicularis oculi

Explanation: ***Sympathetic innervation*** - The **levator palpebrae superioris** muscle, which elevates the eyelid, has dual innervation: the oculomotor nerve for its main portion and **sympathetic innervation** for a smaller auxiliary portion (Müller's muscle). - In oculomotor nerve injury, the complete loss of levator palpebrae superioris function would lead to complete ptosis. However, if partial ptosis is observed, it indicates that the **sympathetic innervation** to Müller's muscle is still intact, providing some eyelid elevation. *Supply from opposite oculomotor nerve* - The oculomotor nerves are distinct and **do not cross-innervate** the levator palpebrae superioris muscles in such a way that one can compensate for injury to the other. - Each oculomotor nerve supplies its respective ipsilateral muscles; there is no functional compensation from the contralateral nerve in case of injury. *Parasympathetic innervation* - Parasympathetic innervation from the oculomotor nerve is primarily responsible for **pupillary constriction** (via the sphincter pupillae) and lens accommodation (via the ciliary muscle). - It does **not directly contribute** to eyelid elevation, so its integrity would not explain partial ptosis in oculomotor nerve injury. *Action of orbicularis oculi* - The **orbicularis oculi muscle** is responsible for eyelid closure, not elevation. It is innervated by the **facial nerve (CN VII)**. - Intact orbicularis oculi function would lead to normal eyelid closure, but it would not prevent ptosis caused by a damaged levator palpebrae superioris.

Question 370: In cardiac muscles, T-tubules are present at?

A. Z lines (Correct Answer)
B. A lines
C. I lines
D. A-I junction

Explanation: ***Z lines*** - In **cardiac muscle**, the T-tubules are located at the level of the **Z lines**, which delineate the sarcomere boundaries. - This placement allows for efficient excitation-contraction coupling by bringing the **action potential** into close proximity with the sarcoplasmic reticulum. *A lines* - The **A line** represents the length of the **myosin (thick) filaments** within the sarcomere. - T-tubules are not typically found at the A line; their primary function requires them to be positioned at the ends of the sarcomere. *I lines* - The **I line** (or I band) contains only **actin (thin) filaments** and spans from the edge of the A band to the Z disk. - While I lines are adjacent to Z lines, the precise location for T-tubules in cardiac muscle is at the Z line itself, not the I line. *A-I junction* - The **A-I junction** is the region where the A band (myosin) meets the I band (actin). - In **skeletal muscle**, T-tubules are located at the A-I junction, but in **cardiac muscle**, they are specifically at the Z line.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

Practice Questions

Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

Practice Questions

Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

Practice Questions

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